The substantive definition of the family is based on blood, meaning shared genetic heritage, and law, meaning social recognition and affirmation of the bond, including both marriage and adoption. The state of being related to others is called kinship. The family and the kin group are not necessarily one and the same. The United States follows the pattern of bilateral descent, which means that both sides of a person's family are regarded as equally important. Patrilineal descent indicates that only the father's relatives are important in determining property, inheritance, and emotional ties. In societies that favor matrilineal descent, only the mother's relatives are significant.

An extended family is a family in which relatives (e.g., grandparents, aunts, or uncles) live in the same home as parents and their children. The nuclear family includes a married couple and their unmarried children living together. Most people in the United States view the nuclear family as the preferred family arrangement. Family types are relevant to various forms of marriage, which include: monogamy, serial monogamy, and polygamy (which includes polygyny and polyandry).

The functionalist definition of the family focuses on what families do for society and their members. William Ogburn identified six primary functions: (1) reproduction; (2) socialization; (3) protection; (4) regulation of sexual behavior; (5) affection and companionship; (6) provision of social status.

Authority patterns within families have often been shaped by gender. A society that expects males to dominate in all family decision making is termed a patriarchy. By contrast, in a matriarchy, women have greater authority than men. Within the egalitarian family, spouses are regarded as equals; however, wives may hold authority in some spheres and husbands in others.

Mate selection is influenced by distinctive cultural norms and values. Many people now use the Internet for matchmaking services. In many traditional societies, parents still arrange marriages for their children. Many societies have explicit or unstated rules that define potential mates as acceptable or unacceptable. Endogamy specifies the groups within which a spouse must be found and prohibits marriage with others. Exogamy requires mate selection outside certain groups, usually one's own family or certain kinfolk. The incest taboo prohibits sexual relationships between certain culturally specified relatives. Homogamy—the conscious or unconscious tendency to select a mate with personal characteristics similar to one's own—also influences the selection of marriage partner.

Within the United States, social class, race, and ethnicity create variations in family life. Social class differences are less striking today than they once were. The subordinate status of racial and ethnic minorities profoundly affects family life in the United States. The African-American family suffers from many negative and inaccurate stereotypes. Mexican Americans have traditionally placed proximity to their extended families above other needs and desires.

Caring for children is a universal function of the family, yet the ways in which this care is assigned to family members vary significantly. Parenthood is one of the most important social roles in the United States. A recent extension of parenthood involves adult children continuing to live at home or returning home after college or divorce. This phenomenon is sometimes called the "boomerang generation" or the "full-nest syndrome" in the popular press. In 2004, 9 percent of all children in the U.S. lived in a home with a grandparent. About 4 percent of all people in the United States are adopted. The expectation that a family consists of a wage-earning husband with a wife who stays at home has largely given way to the dual-income household. The diminishing of the "unwed mother" stigma has contributed to more single-parent families. In 2004, a single parent headed about 20 percent of White families with children under 18, 29 percent of Hispanic families, and 59 percent of African-American families. Approximately 45 percent of all people in the United States will marry, divorce, and then remarry. The rising rates of divorce and remarriage have led to a noticeable increase in stepfamily relationships. In the United States and many other countries, overall divorce rates began to increase in the late 1960s but then leveled off; since the late 1980s, the divorce rate has declined by 30 percent. Perhaps the most important factor in the increase in divorce over the last 100 years has been the greater social acceptance of divorce.

One of the most dramatic social trends in recent years has been the tremendous increase in male-female couples who choose to live together without marrying. This practice of cohabitation rose sixfold in the 1960s and increased another 72 percent between 1990 and 2000. More and more people are postponing entry into marriage. However, fewer than 4 percent of women and men in the United States are likely to remain single throughout their lives. There has been a significant increase in childlessness in the United States. About 16 to 17 percent of women will now complete their childbearing years without having borne any children, compared to 10 percent in 1980. The lifestyles of gay and lesbian couples can be varied. The issue of gay marriage is highly controversial. Many local jurisdictions have sought to address the lack of federal and state rights by passing domestic partnership legislation.